Binding of anionic Pt(ii) complexes in a dedicated organic matrix: towards new binary crystalline composites

Abstract

The square-planar [PtX₄]²⁻ complexes (X = Cl, Br) were successfully incorporated into preprogrammed hybrid organic–inorganic systems, exploiting their expected strong anion–π interactions with π-acidic hexaazaphenylenehexacarbonitrile, HAT(CN)₆. The formation and properties of {[PtCl₄]²⁻; HAT(CN)₆} aggregates in MeCN solution were evaluated based on their UV-Vis spectra to reveal the approximate binding constant K_CT = 7.9(2) × 10² dm³ mol⁻¹, molar absorption coefficient ε_CT = 1.47(2) × 10³ dm³ mol⁻¹ cm⁻¹, extent of electronic coupling H_CT = 2.18 × 10³ cm⁻¹, and electron delocalization α² = 1.75 × 10⁻² (α = 0.13). Strong [PtCl₄]²⁻⋯HAT(CN)₆ interactions in such adducts were also confirmed by the distinct shifts |Δδ_iso| = 0.4 ppm of ¹³C NMR peaks, when compared to the π-acid alone. The crystal structures of the resulting (PPh₄)₂[PtX₄][HAT(CN)₆]·3MeCN (1-Cl⁻ and 1-Br⁻) solids are isomorphous with (PPh₄)₂[Pt(CN)₄][HAT(CN)₆]·3MeCN (1-CN⁻) reported by us previously. The halogenoplatinates occupy exactly the same nodes in the supramolecular network as cyanoplatinate, forming stacked {[PtX₄]²⁻;HAT(CN)₆}_∞ columns that are stabilized by [PPh₄]⁺ cations. However, contrary to the pale yellow coloration of the [Pt(CN)₄]²⁻/HAT(CN)₆ systems, currently the dark violet or dark green coloration of solutions and crystalline phases were noted owing to the intense absorption in almost the whole visible region. DFT calculations reproduced the UV-Vis spectroscopic characteristics and linked it with the enhanced charge-transfer of the [PtX₄]²⁻⋯HAT(CN)₆ electronic interactions. Based on the isomorphism of all three (PPh₄)₂[PtL₄][HAT(CN)₆]·3MeCN congeners we constructed and characterized the unprecedented, first ever anion–π-based binary rod-like core–shell crystalline composites 1-X@1-CN.